In general, the known methods for monitoring traffic areas differ in that different types of sensors are used, which sensors are either embedded beneath the roadway surface or are disposed above the roadway surface. Sensors that are embedded beneath the roadway surface are associated with high installation costs and must be replaced from time to time as the roadway surface wears down over time.
According to EP 1 662 272 A1, instead of embedding a sensor at a predetermined site in each individual lane, a radar beam of low beam expansion which is emitted by a radar antenna is directed at a circumscribed observation site to detect automotive vehicles as they pass the observation site. The detection can be used to activate a red-light camera, to detect the speed of a vehicle or to measure the volume of traffic.
Based on the knowledge of the distance between the observation point and the transmitting and receiving site of the radar beam, compared to the distance derived from the reflected signals, it can be readily determined when a vehicle passes the observation point in order to generate an activating signal. To avoid incorrect detection, it is proposed that a series of distances be derived from a large number of reflected signals and to accept the detection of the vehicle at the observation site as valid only if the values of the distances agree with one another.
The reception of a large number of reflection signals is based on the assumption that the observation site is not a point that a vehicle passes but is an extended area which is described by a large number of distances, all of which are located within a distance range. Therefore, the applicant believes that instead of using the term observation site, it would be more appropriate to use the term observation area, which will in fact be the term used hereinafter to describe the invention.
Since according to EP 1 662 272 A1 the radar beam is directed in a circumscribed manner at an observation area that is circumscribed within a lane, each detected distance must be linked with the associated lane, which lane uniquely identifies the measured vehicle.
As the drawings of EP 1 662 272 A1 indicate by way of an example, the observation area used in the method for detecting a red-light violation is limited to 2 m and includes the stop line.
Since, as proposed, an activating signal, which triggers a camera connected to a traffic light, is generated when a vehicle is detected within the observation area, it is ensured that an image is captured when a vehicle passes through the observation area, and thus crosses the stop line, within a specific switch phase of the traffic light, e.g., only within the red-light phase.
The rules to prove traffic violations differ from country to country. For example, there are countries in which a first image must be taken several meters before the stop line in order to prove a red-light violation.
This might suggest to the person skilled in the art that the observation area therefore be circumscribed before the stop line. However, this solution would have the drawback that a picture of a vehicle is recorded even if the vehicle comes to a stop before the stop line.
Another drawback of a method according to EP 1 662 272 A1 is that in order to detect traffic violations, each lane requires a radar antenna and, moreover, each radar antenna must be oriented separately. The investment cost for the traffic areas to be monitored increases with the number of the lanes to be monitored.